The invention is related to stretch wrap films that are used extensively in packaging to package discrete units together to form a unitary package and are also frequently used to attach a package to a palette, for example. Stretch wrap film may also be used as wrapping to protect a commodity from the environment during handling and transport.
Stretch wrap may be applied by an automatic or semiautomatic machine which includes stretch rollers to pre-stretch the film before it is wrapped around a package. Alternatively the stretch wrap may be applied manually by applying the film from a roll supported on a simple handheld mandrel. The present application has particular application to this latter category of stretch wrap material, and also to film for use on machines in which the amount of stretch prior to wrapping is low or zero.
EP-A-0531021 describes a process for producing a stretch wrap plastic film in which the film is xe2x80x9ccold stretchedxe2x80x9d at ambient temperatures to thereby plastically and elastically stretch the film to cold orient it. The cold oriented film is then allowed to relax to recover substantially its elastic deformation before being formed into a roll. This arrangement provides a film wrap which is substantially as economical, and may be more economical, in film usage when applied by a hand wrapping device than other stretch wrapping films are when applied by automatic and semi-automatic machinery including power pre-stretching devices. The process described in this patent specification has been commercially successful but, in practice, it has not been possible to achieve consistently as high a stretch ratio as 1:4 suggested in the patent specification without experiencing difficulties due to film breakage.
According to this invention a method of making a plastics stretch film comprises the steps of taking a cast or blown film of LLDPE at a temperature of between 50xc2x0 C. and 100xc2x0 C., stretching it in two successive stretching steps, the first step having a stretch ratio higher than that of the second step, to cause both plastic and elastic deformation of the film, relaxing the stretched film substantially to release all of the elastic deformation and winding the substantially relaxed film into rolls.
Preferably, stretching is performed at a temperature of between 75xc2x0 C. and 90xc2x0 C.
By having a film in the temperature range specified above and stretching it in two successive stages it has been found that it is possible to produce more consistently a pre-stretched film. The film also has advantages in having a clearer, less opaque appearance.
Preferably the stretching of the film that occurs in the two successive steps has a stretch ratio in a range from 1:1.5 to 1:2.5 for each step. More preferably, the first stretch ratio is in the range 1:1.85 to 1:1.95 and the second stretch ratio is in the range 1:1.70 to 1:1.80. Preferably during the relaxing step there is a reduction ratio of substantially 1:0.85 between the speed of the film during the second stretch rolling step and the speed of the film during wind-up.
Preferably the temperature of the stretch film is substantially 80xc2x0 C. during the stretching steps. The plastics film may be at this temperature by carrying out the stretching steps at an appropriate position downstream of a casting or blowing/extrusion production line so that the method in accordance with the present invention is carried out in line with the film production. Preferably however the method of the present invention is carried out out of line with the basic film production process and, in this case, the film is preferably re-heated, for example by being passed over one or more heated rollers immediately before the stretching steps.
The plastics material particularly preferred for the stretch film of the present invention is linear low density polyethylene (LLDPE). This material is a copolymer of ethylene with a C4 to C10 olefin; for example, butene-1, 3-methyl-butene-1, hexene-1, 3-methyl-pentene-1, 4-methylpentene-1, 3-methyl-hexene-1, octene-1, decene-1 or a mixture thereof. The alpha-olefin is usually present in an amount from 1 to 10 weight percent of the copolymer. A typical manufacturing process for the formation thereof is disclosed in U.S. Pat. Nos. 4,076,698 and 4,205,021. The preferred LLDPE has a density ranging from 0.900 to 0.940 g/cm3. This material preferably has a melt index of from 1 to 6. A multilayer film is also suitable, such as a multilayer film having a three layer A-B-C structure wherein the A layer comprises LVLDPE, the B layer comprises metallocene LLDPE and the C layer comprises LMDPE.